1. Field of the Invention
The present invention relates to a variable resistor used in devices such as a hearing aid, a measuring instrument, communications equipment, a sensor, or other such electronic apparatus, and in particular, the present invention relates to a miniaturized variable resistor.
2. Description of the Related Art
In recent years, reduction in the size and the weight of electronic equipment has been desired, and the reduction in the size and the weight of the circuit components provided within such electronic equipment has also been desired. An ultra-miniature variable resistor having a diameter or side length of about 2 mm has been used as a circuit component. A variable resistor requires a slider which is a spring component. However, the miniaturization of the slider makes it difficult to achieve good electrical contact and sliding characteristics.
FIGS. 1 and 2 show a conventional ultra-miniature variable resistor having a structure in which a shaft 2, a rotor 3, a slider 4, and a substrate 5 are provided in a case 1 and which are sealed by filling a sealing resin 6 such as epoxy resin in the bottom opening of the case 1. Lead terminals 71, 72, and 73 are fixed on the substrate 5. The lead terminals are electrically connected to both sides of an arcuate resistor 51 and a collector electrode 52, respectively.
As shown in FIG. 3, the slider 4 is defined by a thin metallic plate having a annular arm portion 41 making sliding contact with the arcuate resistor 51 of the substrate 5 and an I-shaped arm portion 42 arranged to contact the collector electrode 52 of the substrate 5. The annular arm portion 41 is bent up at the portion corresponding to the diameter, and the I-shaped arm portion 42 extends in a direction that is substantially perpendicular to the bent-up line of the annular arm portion 41 and is located inside of the annular arm portion 41. In the left side view of FIG. 3, the solid line designates the slider under a load, and the two-dot chain line designates the slider without a load. In the slider 4, a pair of through holes 44 for fitting the protrusions 31 (see FIG. 4) of the rotor 3 are provided at bilaterally symmetrical positions around the I-shaped arm portion 42. The slider 4 is mounted on the rotor 3 so as to he rotatable together with the rotor 3, by fitting the protrusions into the through holes 44 and then crushing the protrusions 31 by weld-caulking.
As shown in FIGS. 5A and 5B when the substrate is mounted in the variable resistor, both of the annular arm portion 41 and the I-shaped arm portion 42 are subjected to a load. As a result, a moment M in the backward-tilting direction of the slider 4 occurs about the bending-up line 43 (see FIG. 5B). The distance between the the protrusions 31 and the bending-up line 43 define the fulcrum about which the moment M acts. Once a backward-tilting of the slider 4 occurs, a problem arises that the contact between the resistor 51 and the annular arm portion 41 and the contact between the collector electrode 52 and the I-shaped arm portion 42 become unstable, resulting in a reduction in the reliability of the variable resistor.
In order to prevent the slider 4 from backwardly tilting, the protrusions 31 of the rotor 3 are weld-caulked. However, because the two protrusions 31 are arranged in the vicinity of the bending-up line 43, the distance between the protrusions 31 and the bending-up line 43 defining the fulcrum is small. Because the distance defining the fulcrum is small, the backward-tilting of the slider 4 cannot be effectively prevented. Furthermore, in the case of very small components, protrusions 31 having a sufficient size cannot be formed on the rotor 3, and the weld-caulking of the protusions 31 is very difficult. For example, when the diameter of the slider 4 is 1.5 mm, the diameter of the protrusions needs to be about 0.2 mm to effectively prevent the backward-tilting of the slider 4. The process of welding such small protrusions 31 is thus very difficult, and a desired fixing strength cannot be reliably achieved even though welding is executed.
In order to overcome the problems described above, the preferred embodiments of the present invention provide a variable resistor arranged to effectively and reliably prevent a slider from backwardly tilting toward a rotor, thereby maintaining a stable contact between the slider and a substrate.
According to one of the preferred embodiments of the present invention, a variable resistor includes a case, a rotor rotatably accommodated in the case, the rotor being rotationally operated from the outside of the case, a substrate located in the case, the substrate having a collector electrode at an approximate central portion of the surface thereof, and having an arcuate resistor disposed around the collector electrode so as to be substantially concentric therewith, a slider mounted on the rotor so as to be rotatable together with the rotor. The slider having an annular arm portion arranged to achieve sliding contact with the arcuate resistor on the substrate, a substantially I-shaped arm portion arranged to contact the collector electrode, and a base portion that is integral with the slider. The base portion being coupled at one end thereof with the annular arm portion and the substantially I-shaped arm portion by a folded back structure. The base portion extends up to the vicinity of the position corresponding to the tip portion of the annular arm portion. The rear surface of the base portion is supported by the rotor. In this variable resistor, the annular arm portion is bent up at the portion corresponding to the diameter or at the portion in the vicinity of the diameter. The substantially I-shaped arm portion extends in a direction that is substantially perpendicular to the bent-up line of the annular arm portion and is located inside the annular arm portion.
The annular arm portion of the slider rotates and makes sliding contact with the arcuate resistor of the substrate. The substantially I-shaped arm portion makes contact with the collector electrode of the substrate. When both the annular arm portion and the substantially I-shaped arm portion are under a load, a moment about the bent-up line of the annular arm portion and in the backward-tilting direction acts on the slider. In the slider in accordance with a preferred embodiment of the present invention, a base portion is integral with the slider. The base portion is coupled with the annular arm portion and the substantially I-shaped arm portion by a folded back structure. The base portion extends up to the vicinity of the position corresponding to the tip portion of the annular arm portion. With this configuration, the base portion supports the moment in the backward-tilting direction and prevents the slider from backwardly tilting. This stabilizes the contact between the resistor and the annular arm portion and the contact between the collector electrode and the substantially I-shaped arm portion, which leads to an improvement in the reliability of the variable resistor.
Preferably, the annular arm portion, the substantially I-shaped arm portion, and the base portion are coupled such that the base portion is folded back, and the annular arm portion, the substantially I-shaped arm portion, and the base portion are closely contacted by the folded-back structure. Thereby, the height dimension of the slider can be made smaller than the structures of the prior art. This allows variable resistors of the preferred embodiments of the present invention to have lower profiles.
It is preferable that the substantially I-shaped arm portion is raised without folding the substantially I-shaped arm portion. This is accomplished by bending a portion of the folded-back portion in the direction opposite to the bending-up direction of the annular arm portion. In the slider having a conventional structure, because the substantially I-shaped arm portion is bent and raised, work-hardening occurs in the bent portion under the influence of bending work. The work-hardening of the bent portion results in the elastic region in the substantially I-shaped arm portion becoming samller. That is, the effective spring length L1 of the substantially I-shaped arm portion (see FIG. 5A) becomes smaller. This causes the problem that the contact pressure between the substantially I-shaped arm portion and the collector electrode becomes greater than is needed. On the other hand, in the above-described construction in accordance with a preferred embodiment of the present invention, the work-hardening due to bending work is prevented because the substantially I-shaped arm portion is substantially linear, and the raising of the root portion of the substantially I-shaped arm portion by folding is not done. and the stress with respect to the load disperses throughout the substantially I-shaped arm portion. This results in the substantially I-shaped arm portion having a wider elastic region. That is, it is possible to make the effective spring length longer and to set the contact pressure between the substantially I-shaped arm portion and the collector electrode to an appropriate value.
Preferably, a pair of through holes for fitting the protrusions of the rotor are provided in the annular arm portion and the base portion in the vicinity of the folded back portion and substantially at bilaterally symmetrical positions around the substantially I-shaped arm portion. Specifically, the through holes are preferably arranged at common positions relative to the annular arm portion and the base portion. The insertion of the protrusions of the rotor into the through holes of the slider prevents the slider from slipping with respect to the rotor, when the rotor is rotated. Because the slider is prevented from backwardly tilting, there is no need to fix the protrusions by weld-caulking. Also, thin protrusions do not cause any problem because the load applied on the protrusions is small.
The above-described variable resistor in accordance with a preferred embodiment of the present invention may be applied to hearing aids. In hearing aids, variable resistors are used for sensibility adjustment or other adjustment. In accordance with the desire to miniaturize hearing aids, miniature size variable resistors have been required. Use of a variable resistor having stable characteristics allows the reliability of a hearing aid to be greatly improved.
In the present invention, the method for attaching the slider and rotor is not limited to the conventional method in which the protrusions are weld-caulked. This is because the slider would not slip with respect to the rotor because of the spring forces of the arm portions.
The above and other characteristics, elements, features, and advantages of the present invention will be apparent from the following detailed description of preferred embodiments of the invention in conjunction with the accompanying drawings.